U.S. patent number 6,963,059 [Application Number 10/609,686] was granted by the patent office on 2005-11-08 for method and system for optimizing illumination power and integration time in an optical sensing device.
This patent grant is currently assigned to EM Microelectronic-Marin SA. Invention is credited to Gil Afriat, James Harold Lauffenburger, Robert R. Rotzoll.
United States Patent |
6,963,059 |
Lauffenburger , et
al. |
November 8, 2005 |
Method and system for optimizing illumination power and integration
time in an optical sensing device
Abstract
There is described an optical sensing device, a method for
controlling operation of an optical sensing device comprising a
light source for illuminating a surface portion with radiation, a
photodetector device having at least one photosensitive element
responsive to radiation reflected from the illuminated surface
portion, and conversion means for integrating an output signal of
said at least one photosensitive element over time during an
integration period of variable duration, which duration depends on
power of the light source and level of radiation reflected from the
illuminated surface portion. The optical sensing device further
comprises a regulating system for controlling power if the light
source as a function of a comparison between a parameter
representative of the evolution of the integration of the output
signal of the said at least one photosensitive element and at least
one reference value. Regulation is advantageously performed by
timing the duration of the integration period or by determining the
rate of evolution of the integrated signal, comparing this duration
or rate of evolution with at least one reference value and
controlling power of the light source as a function of the result
of the comparison. There is also described an optical pointing
device implementing the above regulation scheme as well as an
optical sensing device exploiting this scheme so as to sense
proximity of the illuminated surface portion with respect to the
optical sensing device.
Inventors: |
Lauffenburger; James Harold
(Colorado Springs, CO), Afriat; Gil (Colorado Springs,
CO), Rotzoll; Robert R. (Green Mountain, CO) |
Assignee: |
EM Microelectronic-Marin SA
(Marin, CH)
|
Family
ID: |
33552263 |
Appl.
No.: |
10/609,686 |
Filed: |
July 1, 2003 |
Current U.S.
Class: |
250/205;
250/214R |
Current CPC
Class: |
G01J
1/32 (20130101); G06F 3/0317 (20130101) |
Current International
Class: |
G06F
3/033 (20060101); G01J 001/32 () |
Field of
Search: |
;250/205,214R
;315/152,156,157,158 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Porta; David
Assistant Examiner: Ko; Tony
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. An optical sensing device comprising: a light source for
illuminating a surface portion with radiation; a photodetector
device having at least one photosensitive element responsive to
radiation reflected from the illuminated surface portion; and
conversion means for integrating an output signal of said at least
one photosensitive element over time during an integration period
of variable duration, which duration depends on power of said light
source and level of radiation reflected from the illuminated
surface portion, said optical sensing device further comprising a
regulating system for controlling the power of the light source as
a function of a comparison between a parameter representative of
the evolution of the integration of the output signal of said at
least one photosensitive element and at least one reference value;
wherein said representative parameter is a rate of evolution of the
integrated output signal of said at least one photosensitive
element, said regulating system comprising: means for determining
said rate of evolution during the integration period; comparator
means for comparing the determined rate of evolution with at least
one reference rate value; and power control means for controlling
the power of the light source as a function of the result of the
comparison between the determined rate of evolution and said at
least one reference rate value.
2. The optical sensing device of claim 1, further comprising a
programmable memory means for storing said at least one reference
value.
3. The optical sensing device of claim 1, wherein said regulating
system comprises means for interrupting integration of the output
signal of said at least one photosensitive element if the duration
of the integration period reaches or is likely to reach a
predetermined timeout value, said regulating system increasing
power of said light source if the duration of said integration
period has reached or is likely to reach the predetermined timeout
value.
4. An optical sensing device comprising: a light source for
illuminating a surface portion with radiation; a photodetector
device having at least one photosensitive element responsive to
radiation reflected from the illuminated surface portion; and
conversion means for integrating an output signal of said at least
one photosensitive element over time during an integration period
of variable duration, which duration depends on power of said light
source and level of radiation reflected from the illuminated
surface portion, said optical sensing device further comprising a
regulating system for controlling the power of the light source as
a function of a comparison between a parameter representative of
the evolution of the integration of the output signal of said at
least one photosensitive element and at least one reference value;
wherein said regulating system controls the power of the light
source so that the duration of said integration period remains
within a reference window having lower and upper reference values,
said regulating system decreasing power of the light source so as
to maintain the duration of said integration period above the
window's lower reference value and increasing power of the light
source so as to maintain the duration of said integration period
below the window's upper reference value.
5. A method for controlling operation of an optical sensing device
having a light source and a photodetector device with at least one
photosensitive element, said method comprising the steps of:
illuminating a surface portion with radiation by means of said
light source; detecting radiation reflected from the illuminated
surface portion with said at least one photosensitive element; and
while said surface portion is being illuminated, integrating an
output signal of said at least one photosensitive element over time
during an integration period of variable duration, which duration
depends on power of the light source and level of radiation
reflected from said illuminated surface portion, said method
further comprising the steps of: determining a parameter
representative of the evolution of the integration of the output
signal of said at least one photosensitive element; comparing the
determined representative parameter with at least one reference
value; and controlling power of the light source as a function of
the result of the comparison between the determined representative
parameter and said at least one reference value; wherein said
representative parameter is the duration of said integration period
or a rate of evolution of the integrated output signal of said at
least one photosensitive element.
6. The method of claim 5, comprising the step of providing a
register the value of which determines the power of said light
source, said step of controlling power of the light source
including adjusting the value of said register.
7. The method of claim 5, wherein the power of said light source is
controlled in a stepwise manner.
8. The method of claim 5, further comprising the steps of:
interrupting integration of the output signal of said at least one
photosensitive element if the duration of said integration period
reaches or is likely to reach a predetermined timeout value; and
increasing the power of said light source if the duration of said
integration period has reached or is likely to reach the
predetermined timeout value.
9. The method of claim 8, further comprising the step of providing
a warning signal if the duration of said integration period has
reached or is likely to reach the predetermined timeout value and
if the power of said light source is at a maximum.
10. The method of claim 8, comprising the step of periodically
activating the optical sensing device at a selected activation
rate, said method further comprising the step of setting said
activation rate to a minimum if duration of said integration period
has reached or is likely to reach the predetermined timeout value
and if the power of said light source is at a maximum.
11. The method of claim 5, wherein said representative parameter is
a rate of evolution of the integrated output signal of said at
least one photosensitive element, power of the light source being
controlled while said light source is activated.
12. A method for controlling operation of an optical sensing device
having a light source and a photodetector device with at least one
photosensitive element, said method comprising the steps of:
illuminating a surface portion with radiation by means of said
light source; detecting radiation reflected from the illuminated
surface portion with said at least one photosensitive element; and
while said surface portion is being illuminated, integrating an
output signal of said at least one photosensitive element over time
during an integration period of variable duration, which duration
depends on power of the light source and level of radiation
reflected from said illuminated surface portion, said method
further comprising the steps of: determining a parameter
representative of the evolution of the integration of the output
signal of said at least one photosensitive element; comparing the
determined representative parameter with at least one reference
value; and controlling power of the light source as a function of
the result of the comparison between the determined representative
parameter and said at least one reference value; wherein the power
of the light source is controlled so that the duration of said
integration period remains within a reference window having lower
and upper reference values, the power of said light source being
decreased or increased so as to maintain the duration of the
integration period respectively above the window's lower reference
value and below the window's upper reference value.
13. An optical pointing device comprising: a light source for
repetitively illuminating a surface portion with radiation; and an
optical sensing unit comprising a photodetector array including a
plurality of pixels responsive to radiation reflected from the
illuminated surface portion, each of said pixels including a
photosensitive element coupled to an integrating circuit for
integrating an output signal of the photosensitive element during
an integration period of variable duration, which duration depends
on power of said light source and level of radiation reflected from
the illuminated surface portion, wherein said optical pointing
device further comprises a regulating system including: means for
determining a parameter representative of the evolution of the
integration of the output signals of the photosensitive elements;
comparator means for comparing the determined representative
parameter with at least one reference value; and power control
means for controlling the power of the light source as a function
of the result of the comparison between the determined
representative parameter and said at least one reference value,
wherein said representative parameter is the duration of said
integration period or a rate of evolution of the integrated output
signals of said photosensitive elements.
14. The optical pointing device of claim 13, wherein said
integrating circuits integrate the output signals of said
photosensitive elements until the integrated output signal of a
most illuminated one of said pixels reaches a predetermined value
or until an averaged signal derived from the integrated output
signals of said pixels reaches a predetermined value.
15. The optical pointing device of claim 13, further comprising a
programmable memory means for storing said at least one reference
value.
16. The optical pointing device of claim 13, further comprising a
register the value of which determines the power of said light
source, said register being adjusted in a stepwise manner as a
function of the result of the comparison between the determined
representative parameter and said at least one reference value.
17. The optical pointing device of claim 13, further comprising
means for interrupting integration of said output signals if the
duration of said integration period reaches or is likely to reach a
predetermined timeout value, said power control means increasing
power of said light source if the duration of said integration
period has reached or is likely to reach the predetermined timeout
value.
18. The optical pointing device of claim 17, further comprising
means for generating a warning signal if the duration of said
integration period has reached or is likely to reach the
predetermined timeout value and if the power of said light source
is at a maximum.
19. The optical pointing device of claim 17, further comprising
means for periodically activating said light source, said optical
sensing unit and said regulating system at a selected activation
rate, and means for setting said activation rate to a minimum if
the duration of said integration period has reached or is likely to
reach the predetermined timeout value and if the power of said
light source is at a maximum.
20. An optical pointing device comprising: a light source for
repetitively illuminating a surface portion with radiation; and an
optical sensing unit comprising a photodetector array including a
plurality of pixels responsive to radiation reflected from the
illuminated surface portion, each of said pixels including a
photosensitive element coupled to an integrating circuit for
integrating an output signal of the photosensitive element during
an integration period of variable duration, which duration depends
on power of said light source and level of radiation reflected from
the illuminated surface portion, wherein said optical pointing
device further comprises a regulating system including: means for
determining a parameter representative of the evolution of the
integration of the output signals of the photosensitive elements;
comparator means for comparing the determined representative
parameter with at least one reference value; and power control
means for controlling the power of the light source as a function
of the result of the comparison between the determined
representative parameter and said at least one reference value;
wherein said power control means control the power of the light
source so that the duration of said integration period remains
within a reference window having lower and upper reference values,
said power control means decreasing power of the light source so as
to maintain the duration of said integration period above the
window's lower reference value and increasing power of the light
source so as to maintain the duration of said integration period
below the window's upper reference value.
21. An optical sensing device for use in a pointing device
comprising: a light source for illuminating a surface portion with
radiation; a photodetector device having at least one
photosensitive element responsive to radiation reflected from the
illuminated surface portion; and conversion means for integrating
an output signal of said at least one photosensitive element over
time during an integration period of variable duration, which
duration depends on power of said light source and level of
radiation reflected from the illuminated surface portion, said
optical sensing device further comprising means for sensing
proximity of the illuminated surface portion with respect to the
optical sensing device, said means including: means for determining
if the duration of said integration period reaches or is likely to
reach a predetermined timeout value; power control means for
increasing power of said light source if the duration of said
integration period has reached or is likely to reach the
predetermined timeout value; and means for detecting if the
duration of said integration period has reached or is likely to
reach the predetermined timeout value and if the power of said
light source is at a maximum, such condition being indicative of
the fact that a distance between the optical sensing device and the
surface portion is greater than an operating distance.
22. The optical sensing device of claim 21, further comprising:
timer means for timing the duration of the integration period; and
comparator means for comparing the timed duration of the
integration period with said predetermined timeout value.
23. The optical sensing device of claim 21, further comprising:
means for determining a rate of evolution of the integrated output
signal of said at least one photosensitive element during
integration; and comparator means for comparing the determined rate
of evolution with a predetermined rate of evolution which
corresponds to a rate of evolution below which it can be predicted
that the duration of the integration period is going to reach said
predetermined timeout value.
24. The optical sensing device of claim 21, wherein said
photodetector device comprises a plurality of photosensitive
elements and wherein said means for determining if the duration of
said integration period reaches or is likely to reach a
predetermined timeout value monitor the evolution of the integrated
output signal of a most illuminated one of said photosensitive
elements or of an averaged signal which is derived from the
integrated output signals of said photosensitive elements.
Description
FIELD OF THE INVENTION
The present invention generally relates to optical sensing devices
comprising a light source for illuminating a surface portion with
radiation, a photodetector device having at least one
photosensitive element responsive to radiation reflected from the
illuminated surface portion and conversion means (or integrating
means) for integrating an output signal of the said at least one
photosensitive element over time during an integration period of
variable duration. Such optical sensing devices are particularly
used in optical pointing devices such as mice, trackballs and other
similar computer peripherals. The present invention also concerns a
method for operating the above optical sensing device as well as an
optical pointing device equipped with the above constituent parts
of the optical sensing device.
BACKGROUND OF THE INVENTION
Optical pointing devices are already known in the art. U.S. Pat.
No. 5,288,993 for instance discloses a cursor pointing device
utilizing a photodetector array and an illuminated target ball
having randomly distributed speckles. U.S. Pat. No. 5,703,356
(related to the above-mentioned U.S. Pat. No. 5,288,993) further
discloses (in reference to FIGS. 23A and 23B of this document) an
optical cursor pointing device in the form of a mouse which does
not require a ball and wherein light is reflected directly from the
surface over which the pointing device is moved.
In both cases, the optical pointing device includes a light source
for repetitively illuminating a surface portion (i.e. a surface
portion of the ball or a portion of the surface over which the
optical pointing device is moved) with radiation and an optical
sensing unit comprising a photodetector array including a plurality
of pixels each having a photosensitive element which is responsive
to radiation reflected from the illuminated surface portion. The
pixels outputs of the photodetector array are typically coupled to
conditioning and processing circuits for tracking and extracting
information about the relative motion between the sensing unit and
the illuminated surface portion.
The technique used in above-cited U.S. Pat. Nos. 5,288,993 and
5,703,356 in order to extract motion-related information is based
on a so-called "Edge Motion Detection" technique. This "Edge Motion
Detection" technique essentially consists in a determination of the
movement of edges (i.e. a difference between the intensity of pairs
of pixels) in the image detected by the photodetector array. Edges
are defined as spatial intensity differences between two pixels of
the photodetector array. The relative motion of each of these edges
is tracked and measured so as to determine an overall displacement
measurement which is representative of the relative movement
between the photodetector array and the illuminated portion of the
surface.
An improved motion detection technique based on the above "Edge
Motion Detection" technique is the subject matter of a pending
international application No. PCT/EP 02/13686 filed on Dec. 3, 2002
(under priority of U.S. provisional application No. 60/335,792 of
Dec. 5, 2001) in the name of the present Applicant and entitled
"Method and sensing device for motion detection in an optical
pointing device, such as an optical mouse" (published under No. WO
03/049018 A1).
In optical sensing devices, it is commonly known to couple a
conversion circuit (or integration circuit) to each photosensitive
element of the photodetector device so as to integrate the output
signals of these photosensitive elements over time during a
so-called integration period. FIG. 1 schematically shows the
general principle of an integrating circuit, designated by
reference numeral 1100, coupled to a photosensitive element, in
this case a photodiode, designated by reference numeral 1000. This
integrating circuit 1100 typically consists of an amplifier 1110
and a capacitive element 1120 (or integration capacitor) connected
between the output and the inverting input of the amplifier, the
photosensitive element 1000 being connected to the inverting input
of the amplifier while the non-inverting input of the amplifier is
tied to a reference potential such as ground. The integrating
circuit 1100 outputs a voltage signal Vout, or integrated signal,
which varies over time and which is in essence the result of the
integration over time of the current signal iout produced by the
photosensitive element 1000. Assuming that current iout has a
substantially constant value during the period where integrating
circuit is active (i.e. during the so-called integration period),
the output voltage Vout will vary substantially linearly over
time.
In some cases, the integration period is set to have a fixed
duration. In some other cases, however, the duration of the
integration period may be variable. This is the case for instance
of the solution described in pending U.S. patent application Ser.
No. 10/001,963 filed on Dec. 5, 2001 in the name of the present
Applicant and entitled "Method, sensing device and optical pointing
device including a sensing device for comparing light intensity
between pixels", which is incorporated herein by reference (this
application is published under No. US 2003/0102425 A1). This
solution is also the subject matter of a pending international
application No. PCT/EP 02/13486 filed on Dec. 3, 2002 under
priority of the above US patent application (this international
application is published under No. WO 03/049017 A1).
The solution described in pending U.S. patent application Ser. No.
10/001,963 basically consists in integrating the output signals of
the photosensitive elements until a predetermined threshold is
reached. Interruption of the integration period can for instance be
performed by monitoring when the integrated signal of the most
illuminated pixel in the photodetector array (i.e. the "brightest"
pixel) reaches the threshold or by monitoring when an averaged (or
summed) signal derived from the integrated signals reaches the
threshold. In both cases, one will understand that the duration of
the integration period is defined by the time taken by the
integrated signal to reach the threshold, which time depends on the
level of light detected by the photosensitive elements. The
duration of the integration period is thus variable.
When applying the above integration scheme in an optical sensing
device or in optical pointing device as defined above (i.e. with
light source, photodetector device and conversion means), one will
understand that the duration of the integration period will depend
on the power of the light source and the level of radiation
reflected from the illuminated surface portion.
Taking account of the fact that the level of radiation reflected
from the illuminated surface portion depends on the optical
properties of the surface, one will understand that the duration of
the integration period may vary greatly as a function of the
reflectivity of the surface. It is however desirable to have a
better and more precise control on the duration of the integration
period and to be less dependent on the type of surface which is
used to reflect the radiation emitted by the light source. In
particular, it is desirable to have a short integration time so as
to ensure higher sensing speed and minimize power consumption of
the optical sensing device. At the same time, it is desirable to
have a sufficiently long integration time so as not to degrade the
functionality of the analog circuitry (in particular the
integrating circuit) of the optical sensing device. It is an object
of the present invention to provide such a solution.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided an
optical sensing device comprising a light source for illuminating a
surface portion with radiation, a photodetector device having at
least one photosensitive element responsive to radiation reflected
from the illuminated surface portion, and conversion means for
integrating an output signal of the said at least one
photosensitive element over time during an integration period of
variable duration, which duration depends on power of the light
source and level of radiation reflected from the illuminated
surface portion, the optical sensing device further comprising a
regulating system for controlling the power of the light source as
a function of a comparison between a parameter representative of
the evolution of the integration of the output signal of the said
at least one photosensitive element and at least one reference
value.
According to one embodiment, the representative parameter is the
duration of the integration period and the regulating system
comprises timer means for timing the duration of the integration
period, comparator means for comparing the duration of the
integration period with at least one reference duration value, and
power control means for controlling the power of the light source
as a function of the result of the comparison between the duration
of the integration period and the said at least one reference
duration value.
According to another embodiment, the representative parameter is a
rate of evolution of the integrated output signal of the said at
least one photosensitive element and the regulating means comprises
means for determining the rate of evolution of the integrated
output signal during the integration period, comparator means for
comparing the determined rate of evolution with at least one
reference rate value, and power control means for controlling the
power of the light source as a function of the result of the
comparison between the determined rate of evolution and the said at
least one reference rate value.
According to a second aspect of the invention, there is provided a
method for controlling operation of an optical sensing device
having a light source and a photodetector device with at least one
photosensitive element, the method comprising the steps of:
illuminating a surface portion with radiation by means of the light
source,
detecting radiation reflected from the illuminated surface portion
with the said at least one photosensitive element, and
while the surface portion is being illuminated, integrating an
output signal of the said at least one photosensitive element over
time during an integration period of variable duration, which
duration depends on power of the light source and level of
radiation reflected from said illuminated surface portion,
the method further comprising the steps of:
determining a parameter representative of the evolution of the
integration of the output signal of the said at least one
photosensitive element,
comparing the determined representative parameter with at least one
reference value, and
controlling power of the light source as a function of the result
of the comparison between the determined representative parameter
and the said at least one reference value.
Again, the representative parameter can be the duration of the
integration period or the rate of evolution of the integrated
output signal of the said at least one photosensitive element.
According to a third aspect of the invention, there is provided an
optical pointing device comprising a light source for repetitively
illuminating a surface portion with radiation, and an optical
sensing unit comprising a photodetector array including a plurality
of pixels responsive to radiation reflected from the illuminated
surface portion, each of the pixels including a photosensitive
element coupled to an integrating circuit for integrating an output
signal of the photosensitive element during an integration period
of variable duration, which duration depends on power of the light
source and level of radiation reflected from the illuminated
surface portion, wherein the optical pointing device further
comprises a regulating system including means for determining a
parameter representative of the evolution of the integration of the
output signals of the photosensitive elements, comparator means for
comparing the determined representative parameter with at least one
reference value, and power control means for controlling the power
of the light source as a function of the result of the comparison
between the determined representative parameter and the said at
least one reference value.
An advantage of the present invention resides in the fact that one
can effectively act, through control of the power of the light
source, on the duration of the integration period and ensure that
this duration remains, in most cases, in the vicinity of a
predetermined reference duration. One therefore has the ability to
somewhat compensate for the changing reflectivity of various
illuminated surfaces. For each type of surface, an optimal light
source power and integration duration is thus found.
Control of the power of the light source also allows to optimise
the power consumption of the optical device. Indeed, the invention
allows selection of the more appropriate light source power to
yield the desired integration duration, i.e. allows optimisation of
the light source power for optimum integration time.
According to a preferred embodiment of the present invention, the
power of the light source is controlled so that the duration of the
integration period remains within a reference window having lower
and upper reference values (advantageously programmable), light
source power being increased so as to maintain the duration of the
integration period below the upper reference value or decreased so
as to maintain the duration of the integration period above the
lower reference value. A reference window is preferable so that the
light source power is not changed too frequently, which could
degrade the device performance.
According to another embodiment of the present invention,
integration of the photosensitive elements can be interrupted if
the duration of the integration period reaches a predetermined
timeout value. At the same time, power of the light source can be
increased. If the timeout condition keeps occurring and the power
of the light source is set at its maximum, this can be interpreted
as being indicative of a "loss of reflection" condition, i.e. that
the distance between the sensing device and the surface is too
great. This "loss of reflection" condition can for instance occur
if an optical mouse implementing the above solution is lifted from
the surface over which it is normally moved. Under such a
condition, the activation rate of the light source, photodetector
device and regulating system may furthermore be set to a minimum
for the purpose of saving power.
According to a fourth aspect of the invention, there is accordingly
also provided an optical sensing device comprising a light source
for illuminating a surface portion with radiation, a photodetector
device having at least one photosensitive element responsive to
radiation reflected from the illuminated surface portion, and
conversion means for integrating an output signal of the said at
least one photosensitive element over time during an integration
period of variable duration, which duration depends on power of the
light source and level of radiation reflected from the illuminated
surface portion, the optical sensing device further comprising
means for sensing proximity of the illuminated surface portion with
respect to the optical sensing device, said means including means
for determining if the duration of said integration period reaches
or is likely to reach a predetermined timeout value, power control
means for increasing power of the light source if the duration of
the integration period has reached or is likely to reach the
predetermined timeout value, and means for detecting if the
duration of the integration period has reached or is likely to
reach the predetermined timeout value and if the power of the light
source is at a maximum, such condition being indicative of the fact
that a distance between the optical sensing device and the surface
portion is greater than an operating distance.
Other aspects, features and advantages of the present invention
will be apparent upon reading the following detailed description of
non-limiting examples and embodiments made with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a conversion circuit coupled
to a photosensitive element for integrating the output signal
thereof over time;
FIG. 2 is a diagram illustrating the basic principle of the
invention;
FIG. 3 is a schematic illustration of an embodiment of an optical
sensing device according to the invention;
FIG. 4 is a diagram exemplifying the evolution over time of
integrated signals under different illumination conditions and
showing possible reference values used as comparison for
controlling the power of the light source;
FIG. 5 is a schematic illustration of an embodiment of an optical
pointing device implementing the invention;
FIG. 6 is a flow chart illustrating a method for controlling
operation of an optical sensing device according to an embodiment
of the invention; and
FIG. 7 is a diagram similar to that of FIG. 4 illustrating a
variation for controlling the power of the light source which is
based on monitoring of the rate of evolution of the integrated
signals.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 illustrates the basic principle of the invention. It
basically consists of an optical sensing system comprising a light
source 10 for illuminating a portion of a surface S with radiation,
a photodetector device 20 having at least one photosensitive
element responsive to radiation reflected from the illuminated
surface portion S, conversion means 30, coupled to the output of
the photodetector device 20, for integrating an output signal of
the said at least one photosensitive element over time during an
integration period of variable duration and a regulating system 40
for controlling the power of the light source as a function of the
duration of the integration period. It should be stressed that the
optical sensing system is designed, within the scope of the present
invention, so that the integration period has a variable duration,
designated Tint, which depends on the power of the light source 10
and the level of radiation reflected from the illuminated surface
portion S. It will thus be appreciated that the optical sensing
system of FIG. 2 includes some sort of feedback loop for enslaving
the power of the light source 10 as a function of the evolution of
the integration.
As this will be understood in the following, the regulating system
40 is used to control (i.e. adjust if necessary) the power of the
light source so that the duration of the integration period
remains, under normal conditions, in the vicinity of at least one
reference duration value. As schematically illustrated in the
example of FIG. 2, three reference values designated Tmin, Tmax and
Ttimeout may be used.
FIG. 3 shows a more detailed block diagram of an optical sensing
device according to one embodiment of the invention. This
embodiment is based on timing of the duration of the integration
period, i.e. the parameter representative of the evolution of the
integration process is the duration of the integration period. It
again includes a light source 10, a photodetector device 20,
conversion means and a regulating system. Light source 10 may be a
light emitting diode (LED) or any other suitable source for
generating radiation within a desired wavelength range. For optical
pointing device, this LED may advantageously be an infrared LED.
The photodetector device 20 is of course to be chosen so as to be
responsive to the radiation emitted by the light source 10 and
should comprise at least one photosensitive element. In practice,
the photodetector device 20 would comprise a plurality of such
photosensitive elements, preferably arranged so as to form a
two-dimensional array. Theoretically, the present principle is
applicable to an optical system having only one photosensitive
element. In FIG. 3, only one photosensitive element is thus shown
for the purpose of explanation.
As already mentioned hereinabove, the conversion means integrate
the output signal of the photosensitive element over time during a
so-called integration period of variable duration. In the
embodiment of FIG. 3, the photosensitive element is coupled to an
integrating circuit, designated by reference numeral 50, the
operating principle of which is similar to that described in
reference to FIG. 1. Timing and resetting of the integrating
circuit 50 is adequately performed by means of a controller 52. The
output of the integrating circuit 50 is coupled to a level detector
54 (or end-of-integration detector) the purpose of which is to
detect when the output of the integrating circuit 50 (the
integrated signal) reaches a determined threshold. When such
condition is detected, level detector 54 outputs an interruption
signal to controller 52, which in turn commands the integrating
circuit 50 to interrupt the integration period. The resulting
integrated signal at the output of circuit 50 is supplied to
processing circuit (not shown in FIG. 3) for further processing and
analysis. This principle basically corresponds to that described in
the already mentioned pending U.S. patent application Ser. No.
10/001,963 filed on Dec. 5, 2001 in the name of the present
Applicant.
In order to time the duration of the integration period, the
optical sensing device of FIG. 3 is additionally provided with a
timer 56 which is coupled to controller 52. This timer 56 is
started, each time the output of photosensitive element 20 begins
to be integrated by the associated conversion means. The output
value of timer 56 will thus be representative of the duration of
the integration period. Controller 52 resets this timer 56 before
each activation period of the system.
The optical sensing device is further provided with a memory means
58 (whether of the volatile or non-volatile type) to store the
reference value or values used for enslaving the power of the light
source 10. The reference values are preferably programmable so as
to allow an eventual adjustment of the operating parameters of the
optical sensing device.
Controller 52 is coupled to light source 10 so as to control its
operation as well as its power characteristics. To this end, a
register 60 is provided for storing a value representative of the
power of the light source to be selected during each flash. The
value of this register 60 is adjusted by controller 52, if
necessary, i.e. either increased, decreased or left unchanged,
according to the duration value outputted by timer 56.
Turning now to FIG. 4, one will briefly illustrate how the power of
the light source could be controlled as a function of the duration
of the integration period according to a preferred embodiment. FIG.
4 is a diagram showing the evolution over time of integrated
signals under four different illumination conditions. Curves a to d
illustrate the evolution of four different integrated signals with
increasing illumination levels. The three reference values Tmin,
Tmax and Ttimeout briefly mentioned in reference to FIG. 2 are
shown on the time axis. On the Y-axis is also shown a value
designated Veoi used as threshold for interrupting integration.
Curves a, b, c illustrate three cases where interruption of the
integration period occurs respectively before Tmin (within range
Tint<Tmin), after Tmin and before Tmax (within range
Tmin.ltoreq.Tint.ltoreq.Tmax), and after Tmax (within range
Tint>Tmax). On the other hand, curve d illustrates a case where
interruption of the integration period occurs at time Ttimeout
before the integrated signal reaches the threshold value Veoi.
In the preferred embodiment of FIG. 4, the duration range between
Tmin and Tmax is chosen to be the target range or window within
which one desires to maintain the duration of the integration
period. Below Tmin, the duration of the integration period is
regarded as being too short, which could degrade the sensor
performance, and above Tmax, the duration of the integration period
is regarded as being too long, which has a negative impact on
sensor speed and power consumption. Within the window Tmin-Tmax,
the duration of the integration period is regarded as adequate.
Adjustment of the power of the light source is thus necessary only
when the duration of the integration period falls outside of the
reference window Tmin-Tmax. The use of a reference window with its
lower and upper limits Tmin, Tmax is preferable so as not to change
the power of the light source too frequently, which could also
impair the sensor performance.
Below Tmin, one will understand that too much radiation is
reflected from the illuminated surface and that the power of the
light source should accordingly be decreased so as to compensate
for this too high illumination. Of course, one assumes that the
illumination level detected by the photodetector device is mostly
dependent on the level of radiation emitted by the light source and
reflected from the illuminated surface portion and that this
illumination level is not mainly due to any other external source.
It should however be mentioned, that if the decrease in power of
the light source does not result in the expected increase of the
duration of the integration period, this could be used as being
indicative of a perturbation due to a parasitic source (such as
ambient light or any other external source of radiation within the
operating wavelength range) located in the vicinity of the
photodetector device.
In contrast to the above situation, above Tmax, the level of light
reflected from the illuminated surface portion is considered to be
too low and the duration of the integration period therefore too
long. Power of the light source should therefore be increased in
order to reduce the duration of the integration period so that it
again falls within the targeted window.
The use of the third reference value Ttimeout is useful in order to
achieve the following objectives. Under some exceptional
conditions, the level of light detected by the photodetector device
can be so low that it would be unacceptable (mainly for reasons of
sensor speed and power consumption) to let the conversion means
integrate the output signal of the photodetector device until
threshold Veoi. Such condition may occur for instance if no more
light is reflected from the surface portion (the optical sensing
device being for instance lifted from the illuminated reference
surface). An extreme limit, or timeout value, is thus defined by
Ttimeout above which no more integration should occur. In contrast
to normal situations where the signals are integrated till they
reach threshold Veoi, integration is interrupted before threshold
Veoi is reached as soon as the duration of the integration period
reaches the timeout limit Ttimeout.
Referring again to the embodiment of FIG. 3, as soon as the value
of timer 56 reaches the timeout value Ttimeout, controller 52
interrupts operation of integrating circuit 50. In addition, the
controller 52 further increases the power of light source 10 by
adjusting register 60. If the timeout condition keeps occurring and
the power of the light source 10 ultimately reaches its maximum
after several successive flashes (which maximum is determined by
the absolute limits of the light source and its driver), this can
be held to be indicative of a "loss of reflection" condition (e.g.
"the optical mouse has been lifted from the surface"). This
condition can further be transmitted and outputted to the user or
host system to which the optical sensing device is connected. One
will therefore understand that there is thereby provided a means
for sensing proximity of the sensing device with respect to the
surface portion which is to be illuminated.
In addition, should the "loss of reflection" condition be detected
(i.e. Tint>Ttimeout and light source power at its maximum), it
is advantageous to further act on the activation rate of the
optical sensing device. Indeed, as already mentioned, the optical
sensing device (namely the light source, the photodetector device,
the conversion means and the regulating system) is typically
activated at a selected activation rate and during a selected
activation period (which activation period is longer that the
integration period). If the "loss of reflection" condition is
detected, the activation rate can thus be decreased to a minimum
for the purpose of saving power. This minimum should be selected
with regard to the level of power consumption that can be saved and
with regard to the time that would be taken by the system to detect
that reflection from the illuminated surface has been
re-established. Further, reporting of motion information from the
optical pointing device may be suspended.
Turning to FIG. 6, one will briefly describe a preferred operation
of an optical sensing device within the scope of the invention
which mostly summarizes the different elements that have been
described hereinabove. FIG. 6 shows a flowchart of operations which
could be undertaken during each flash (or activation period) of the
optical sensing device. This flowchart is applicable in particular
to the optical sensing device of FIG. 2 as well as to the optical
pointing device of FIG. 5 which will be described hereinafter.
Following the start of the flash, the first step S1 of FIG. 6
consists in activating the light source. This activation is made
with consideration of the power settings that may be stored in an
associated register as already mentioned. Next, at step S2,
integration of the output signal of the photodetector device starts
as well as the timing operation of the duration of the integration
period.
At step S3, it is monitored whether the duration that is timed Tint
reaches the timeout value Ttimeout. In the affirmative, the process
continues at step S9. In the negative, the process continues at
step S4 where it is checked whether the end of integration (E.O.I.)
condition has been detected. As long as duration Tint has not reach
the timeout value and end of integration has not been detected,
steps S3 and S4 are continuously performed.
If the end of integration condition is detected at step S4,
integration and timing operations are interrupted and the light
source is deactivated at step S5. Duration Tint is compared at step
S6 with the lower and upper reference values Tmin and Tmax of the
target window. Power of the light source is either decreased at
step S7 if Tint<Tmin, left unchanged if
Tmin.ltoreq.Tint.ltoreq.Tmax, or increased at step S8 if
Tint>Tmax. Steps S7 and S8 may advantageously consist of
decrementing and respectively incrementing the power register,
adjustment being performed in a stepwise manner.
If the timeout condition is detected following the comparison of
Tint and reference value Ttimeout at step S3, integration and
timing operation are interrupted and the light source is
deactivated at step S9. Next, it is checked at step S10 whether
power of the light source is already at its maximum. In the
negative, the process proceeds to step S8 to increase the power of
the light source. In the affirmative, as already mentioned, it is
held at step S11 that a "loss of reflection" condition has
occurred. Next at step S12, the activation rate of the system is
adjusted to a minimum for the purpose of saving power.
The process of FIG. 6 is repeated in a similar manner during each
activation period of the system. The flowchart of FIG. 6 is of
course purely illustrative and shall not be considered as being a
limitation of the scope of the invention. The steps may be modified
in various aspects. Steps S11 and S12 are for instance optional and
additional steps may be provided. For example, provided that the
activation rate is adjusted at step S12, additional steps would be
necessary to detect if reflection has been re-established. This
could easily be performed by providing readjustment of the
activation rate to its nominal value after the end of integration
condition is detected at steps S4 and S5.
Instead of adjusting the power of the light source at the end of
the activation period, power control may alternatively be performed
"on the fly" while the light source is activated. This could be
achieved provided the controller is adapted to monitor the rate of
evolution of the integrated signals. If the integrated signals
(averaged signal or maximum signal) increase too slowly or too
quickly, this might be recognized fast enough to increase or
respectively decrease the light source power while the light source
is on. More specifically, as illustrated by the diagram of FIG. 7,
this could be performed by determining the rate of evolution at a
time, designated Tr before lower reference value Tmin. Since the
evolution of the integrated signal may be assumed to be
substantially linear (the illumination conditions being essentially
constant during one activation period of the light source), one can
predict, based on the slope of the curve of the integrated signal,
the ultimate duration of the integration period and estimate
whether it is going to remain or not within the targeted reference
window. In contrast to the previous embodiment, this allows power
adjustment of the light source while it is activated.
It will be appreciated that the same principle may be adopted in
order to determine whether a timeout condition is likely to occur.
In particular, one can compare the rate of evolution of the
integrated output signal with a predetermined rate of evolution
which corresponds to a rate below which it can be identified and
predicted that the duration of the integration period is ultimately
going to reach the predetermined timeout value Ttimeout. This zone
is identified as the "TIMEOUT RANGE" in FIG. 7. One may either
decide to interrupt integration if such condition occurs or adjust,
namely increase, the light source power if this is still possible.
Again, in case the power of the light source is set to a maximum,
one may exploit this method to implement a proximity sensor.
Turning now to FIG. 5, one will describe an embodiment of an
optical pointing device which implements the regulation scheme
based on timing of the duration of the integration period. The
components that are essentially similar to those of the embodiment
of FIG. 2 are designated by the same references, namely the light
source 10, the controller 52, the end of integration detector 54,
the timer 56, the memory means 58 and the register 60. In contrast
to the embodiment of FIG. 2, the embodiment of FIG. 5 is
specifically adapted for a use in an optical pointing device such
as an optical mouse or trackball. This embodiment thus comprises an
optical sensing unit 70 comprising a photodetector array including
a plurality of pixels 71 responsive to radiation reflected from the
surface portion S. Each pixel includes the arrangement of a
photosensitive element coupled to a corresponding integrating
circuit. Each pixel configuration may essentially be similar to
that shown in FIG. 1. The pixel outputs are fed to the end of
integration detector 54 as well as to a comparator array 80.
Comparator array essentially consists of a plurality of comparator
circuits which are used to extract edge information data from the
pixel outputs, i.e. data that is subsequently exploited by the
motion processing circuitry (not shown) according to the so-called
"Edge Motion Detection" technique briefly mentioned in the preamble
of the specification. This specific circuit configuration is part
of the subject matter of pending international application No.
PCT/EP 02/13686 (Published International Application No. WO
03/049018) filed on Dec. 3, 2002 which has been mentioned hereabove
and will not be described here again.
In contrast to the embodiment of FIG. 2, the end of integration
detector 54 is designed to monitor the outputs of all pixels. As
soon as end of integration is detected, the controller 52
interrupts integration of all integrating circuits within
photodetector array 70 simultaneously. End of integration may be
detected in essentially two ways. A first solution consists in only
monitoring the integrated signal provided by the brightest pixel in
array 70, i.e. the pixel which is the most illuminated, and detect
when this integrated signal reaches the threshold Veoi. Another
solution consists in averaging all pixel outputs and detecting when
the resulting averaged signal reaches the threshold Veoi.
The embodiment of FIG. 5 essentially behaves in a similar manner to
that of FIG. 2. Namely, upon detection of the end of integration
condition by detector 54, controller 52 interrupts operation of the
integrating circuits and then compares the timed duration Tint of
the integration period provided by timer 56 with reference values
Tmin and Tmax stored in memory 58. According to the result of this
comparison, the power of the light source 10 is either decreased if
duration Tint is lower than Tmin, increased if duration Tint is
greater than Tmax, or left unchanged if duration Tint is within the
targeted window Tmin-Tmax. Power is again adjusted through a
stepwise adjustment of register 60. For the next flash, light
source 10 is operated according to the value of register 60.
In addition, while the integrating circuits are operating,
controller 52 also monitors the timed duration supplier by timer 56
and compares it with the third reference value, or timeout value,
Ttimeout. If timeout occurs, then controller 52 commands the
integrating circuits to interrupt integration (controller 52 also
deactivates the light source) and increments register 60 for the
next flash. If the power settings of the light source 10 are
already at maximum, controller 52 advantageously generates a "loss
of reflection" warning signal and, eventually, decreases the
activation rate of the system.
Having described the invention with regard to certain specific
embodiments, it is to be understood that these embodiments are not
meant as limitations of the invention. Indeed, various
modifications and/or adaptations may become apparent to those
skilled in the art without departing from the scope of the annexed
claims. For instance, the proposed embodiments are not necessarily
limited to devices comprising a light emitting diode as light
source or photodiodes as photosensitive elements. Any other
suitable light source and photosensitive element may be used.
In addition, as already mentioned, adjustment of the power of the
light source may either be performed at the end of each activation
period (or "flash") or "on the fly" while the light source is
activated and the conversion means are still running.
* * * * *